The present invention relates to brake systems which operate according to the return delivery principle.
In order to adjust optimal slip on the wheels in these systems, pressure fluid must be supplied to the wheel brake cylinders and removed again therefrom according to a control algorithm when a brake slip control action (anti-lock controlled braking action) is performed. For pressure build-up, pressure fluid is delivered out of the master brake cylinder into the wheel brakes. For pressure decrease, this pressure fluid is supplied back into the master brake cylinder by means of a pump.
It also is known to use this hydraulic circuitry for controlling traction slip. For this purpose, a valve is switched over, and as a result the master brake cylinder is uncoupled from the brake line and a pressure accumulator is connected for traction-slip-controlled braking action. For example, in order to avoid excessive spinning of the wheels when starting to drive, pressure fluid is fed out of the accumulator to the wheel brakes, whereby the starting torque of the engine is compensated by a brake torque. For pressure decrease, the pressure fluid is returned from the wheel brakes into the accumulator by means of the pump.
Theoretically, these systems provide a closed pressure fluid system, since no new pressure fluid must be introduced. In fact, however, there is increased pressure fluid requirement due to leakage in the lines as well as due to pad wear, the result is that the accumulator must be charged in certain time intervals. To achieve this, systems have been suggested as in German patent publications DE-OS 31 37 286 and DE-OS 31 37 287. It is proposed therein that the driver is informed of the low charge condition accumulator. By this, the driver is induced to actuate a switch for issuing an activating signal to the solenoid valves, whereby a pressure fluid connection is established between master brake cylinder and accumulator in order to apply the brake. The volume which is displaced during this action out of the master brake cylinder is supplied into the accumulator by way a non-return valve. The driver can be assisted in that the master brake cylinder is connected by way of a shunt line to the inlet of the pump so that in addition to the pedal force, the suction force of the pump is available for charging the accumulator.
This method is susceptible to malfunction because the driver is called upon to interact. This is because in case the driver does not respond, the pressure in the accumulator remains on a level which does not provide a proper traction slip control action. Moreover, it may become necessary to apply the brake several times, since the fluid volume in the master brake cylinder is insufficient to charge the accumulator.
Further, German patent publication P-34 39 408 suggests supplying fluid by means of a pump by way of the shunt line, when the brake is not applied, out of the supply reservoir allocated to the master brake cylinder into the accumulator. This arrangement has the disadvantage that the valve which connects the accumulator instead of the master brake cylinder to the brake line must be actuated for charging the accumulator. An instability factor ensues therefrom, since the valve will not move into its initial position when switched back and that the connection between the master brake cylinder and the wheel brake remains interrupted. Braking therefore would not be possible.